Coal

Coal fueled the industrial revolution that led to everything from planes, trains, and automobiles to cheaply produced food, cheaply constructed apartment buildings, mass produced housing, and most everything else we see in the United States today. Coal continues to be the most important source of energy when it comes to electricity, so the wheels of industry and your refrigerator are still largely powered by coal. Coal is also geologically interesting. If you find a layer of coal, you can be pretty sure that you are looking at the pressure-cooked remains of an ancient swamp. Coal starts out as peat. Upon burial, the peat is gradually compressed and heated until it forms lignite coal. Further burial produces bituminous coal, which is the one most commonly used to generate power. Metamorphic coal is called anthracite (meaning 'black rock'). The photos below are all bituminous coal.

Fresh coal is jet black. When exposed to weathering, tiny pyrite crystals found in many coals begin to oxidize. The sulfur in pyrite colors the coal with yellowish bands. The oxidized iron makes rust. Coal often breaks into blocky pieces, as seen here. It also disintegrates pretty quickly!
Coal often breaks into blocks because it develops two sets of intersecting fractures (called 'joints' in geology, 'cleats' in coal mining). This is a top view of a block that fell down a cliff and after a few short years began to fall apart along its fractures.
If you see a dark gray or black sedimentary layer in an outcrop, it is probably either coal or black shale. Black shales are rich in organic matter and clay minerals. Most coals in the eastern U.S. are 6 to 18" thick. The Pittsburgh Coal, shown here, is a remarkable 10' thick.
As you approach an outcrop of coal, the blocky nature of the weathered outcrop becomes clear. Again, the outcrop is not jet-black like fresh coal due to weathering.

More coal!

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